One of the current trends in the automobile industry is to lower vehicle weight to help achieve better fuel economy, thus helping to meet fuel economy standards and to offset the higher fuel prices. Another trend is that there is a broader range of vehicle models, which in turn reduces the volume of vehicles produced on a per model basis.
SUV vehicles remain popular and typically include fairly heavy rear liftgates making this part of the vehicle a target area for weight reduction. Liftgates are traditionally made from stamped steel panels that are heavy and have a high tooling cost. Sheet Molding Compound (SMC) is an alternative to steel for the inner and outer panels of the liftgate. Using SMC has several manufacturing concerns related to the material and process. Steel and SMC liftgates have a mass penalty over thermoplastics. There are also styling restrictions with traditional sheet metal components. Thermoplastic composite type materials used for liftgate applications also have difficulty meeting customer performance specifications.
Another concern with the manufacture of liftgates is that typical liftgates are manufactured as a relatively flat or smoothly contoured panel, with structural reinforcements such as ribs being added onto the panel. This will also add weight and increase manufacturing complexity as well and if thermoplastics are used there are read through areas where the ribs are placed which must be dealt with by design modifications or expensive processes such as gas assist injection molding.
Recently magnesium inner reinforcement panels have been used with an outer polymer skin in order to reduce weight. While such panels are an improvement in weight, this is an expensive solution. Accordingly, there exists a need for a composite liftgate which is both lightweight, as well as structurally sound enough to meet various load requirements, while being more mass and cost effective than magnesium reinforced liftgates.